1. Field of the Invention
The present invention is directed to the field of routing data, including the routing of data in a data processing system or communications network.
2. Description of the Related Art
In data processing environments, multiple data processing engines often require access to the same data. The processing engines pass the data among themselves, until all processing involving the data is complete. In some instances, the processing engines reside in separate systems and transfer data over a communications network. Alternatively, the processing engines all reside in a single system and transfer data over an intra-system network or bus.
A cross-bar facilitates the transfer of data between processing engines by routing incoming data to a target destination. Processing engines send data to the cross-bar with a target destination identifier. The cross-bar determines whether the target is coupled to the cross-bar. If the target is coupled, the cross-bar forwards the data. Otherwise, the cross-bar takes no action on the data.
A traditional cross-bar employs a series of switches to link incoming data from an input port to a target output port. At each switch, the cross-bar makes a routing determination, based on the target destination identifier. If the identified target is coupled to the cross-bar, the incoming data is eventually routed through the network of switches to an output port coupled to the identified target.
Traditional cross-bars often require incoming data to travel through several levels of switches before reaching an output port. Traditional cross-bars employ a large number of switches, making design complex. The large number of switches makes it difficult to meet timing goals and physical design criteria. These traditional cross-bars also consume significant power and circuit space. The disadvantages associated with a traditional cross-bar are particularly troubling when implementing the cross-bar on an integrated circuit.
The switching circuitry in existing cross-bars prevents a single output port from simultaneously servicing multiple packets for a single target. If two input ports receive data for the same target, only one set of incoming data will be transferred to the target. The other set of incoming data is blocked. The blocked data must be resent at a later time—unnecessarily utilizing processing resources and delaying data transfer.
Cross-bars also traditionally support only a single target destination on each output port. This inflexibility makes multicasting addressing impossible, unless special multicast circuitry is added. Multicast addressing allows a single destination identifier to address multiple targets. In many circumstances, one processing engines needs to send the same data to multiple processing engines. Multicast addressing allows a processing engine to accomplish this with a single packet transfer. Without multicast addressing, the processing engine performs multiple packet transfers—wasting both processing resources and system bandwidth.
Support for ensuring quality of service is also inefficient in traditional cross-bars. Existing cross-bars require multiple buffer memories for storing different classes of data. This is wasteful when one or more data classes are idle—the buffer memories for the idle data classes go unused.
Traditional cross-bars are constrained by only supporting packets with limited fixed sizes on the input and output. It is the responsibility of the system that uses the cross-bar switch to break-up large packets of data to conform to the supported fixed packet size at the input. On the output side, the system then has to re-assemble the pieces into the original packets. This puts a significant overhead on the system.